daughterboard

(Or "daughter board", "daughtercard", "daughter
card") A printed circuit board that connects to the
motherboard. The daughterboard is typically smaller than
the motherboard.

A daughterdboard often adds to or supports the main functions
of the motherboard, unlike an expansion card which
provides some new function. For example, a post-release
hardware modification might be released as a daughterboard for
soldering onto the motherboard.